In the past, most heating control systems for occupied spaces such as residential dwellings have provided temperature-based control. The comfort they provide has for the most part been adequate. The majority of such systems have an air recirculating system to heat the recirculated air if the space temperature as sensed by a thermostat is below a comfort range and cool the recirculating air if above the comfort range. Humidity control has resulted either from the inherent reduction of humidity which air conditioning units provide, or from water vapor which is added either incidentally or intentionally to the air in or entering the space. It is generally accepted that people within the enclosed space will find relative humidity between approximately 30% and 50% to be comfortable.
In cold weather, even though its relative humidity is very high, outside air has relatively low dew point temperature. The relative humidity of low dew point air decreases when it is heated. If there is significant infiltration of heated outside air into the occupied space during cold weather, the relative humidity of the space may fall to even below 15% if humidity is not added to the space. If humidity becomes too low in an occupied space, there is even the potential for harm as well as discomfort. For example, too low humidity may cause nosebleeds or cracked and bleeding skin which at the very least is uncomfortable. Glued furniture joints may weaken because of too low humidity. Musical instruments such as pianos, harpsichords, guitars, violins, etc. may be damaged or their tuning affected by low humidity. Certain house plants do not thrive if the humidity is consistently below a preferred range. Oil paintings frequently need a minimum humidity to avoid damage to the painted surface.
There are in occupied spaces, sources of water which incidentally increase the humidity of the space. Plants, showers, saunas, cooking, respirating humans and animals, all increase humidity in the occupied spaces. In very cold seasons, these sources are frequently not adequate to raise the humidity sufficiently. Because of this, a frequent practice is to add humidity to the air in occupied spaces either with portable humidifiers or with installed humidification units operating in connection with the heating plant. In many situations, this is adequate to hold the humidification within the closed space to at least close to the desired range. In many situations, relative humidity need not be controlled as accurately as temperature in order to achieve comfort and to avoid harm to people and objects.
There are certain conditions however, where closer control of relative humidity in a space turns out to be important. The condition which we address in our invention here concerns the situation where there are windows exposed to cold outside air. If the interior space dew point temperature (which increases with increasing relative humidity) rises to a temperature above the interior window surface temperature, there will be condensation on the window surface. If interior humidity is grossly excessive in these situations, the condensation may be so great that condensed water will run down the window surface and damage a wood or steel frame in which the window is set. If the outdoor temperature is below freezing and the insulation provided by the window sash is inadequate, frost will form and may even build up to an appreciable thickness over a period of time. There are even cases where solid ice builds up to a thickness so great on the interior glass pane that it breaks. At the very least, condensation will make it difficult to see out of the window. And condensed water running down the window will often cause streaks making it look dirty. Accordingly, we have found it desirable to limit humidity in occupied spaces during cold weather to prevent this condensation.
There are already control systems for apparatus which can measure and control humidity within a heated space. In U.S. Pat. No. 5,351,855 (owned by the assignee of this application) the outdoor temperature is estimated and from that estimation an acceptable humidity level is determined. This level is used to control the setting of a humidistat which controls the operation of a unit for humidifying air in the enclosed space.
Apparatus for controlling both temperature and humidity within an enclosed space requiring addition of heat and humidity to maintain comfort, to preselected temperature and humidity set points respectively, typically includes a plenum where air circulated to and from the enclosed space can be treated. A return air duct is connected to provide air from the space to the plenum. A heated air duct is connected to allow air flow from the plenum to the space. A fan within the plenum extracts air from the space through the return air duct and forces the extracted air through the conditioned air duct into the space. A heating unit operates responsive to a first value of a heating active signal to heat air flowing through the plenum and ceases operating responsive to a second value of the heating active signal. The heating unit has a heat exchanger within the plenum. An air humidification unit operates responsive to a first value of a humidification active signal to humidify air flowing through the plenum and ceases operates responsive to a second value of the humidification active signal. An indoor temperature sensor within the space supplies an indoor temperature signal encoding a value indicative of the internal temperature of the space. A humidity sensor within the space provides a humidity signal encoding a value indicative of the relative humidity for the air within the space. An outdoor temperature sensor provides an outdoor temperature signal encoding a value indicative of the outdoor air temperature. A humidity set point generator provides a humidity set point signal encoding a humidity set point value.